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Put the sweet potato in a large, deep saucepan over a high heat. Cover with boiling water and boil for 10 mins. Meanwhile, put all the paste ingredients in a food processor and blend until very finely chopped. Set aside until needed.
Put the spinach in a large colander in the sink and pour the sweet potatoes and their cooking water over it to drain the potatoes and wilt the spinach at the same time. Leave to steam-dry.
Return the saucepan to the heat (no need to wash it first), then add the oil, followed by the spice paste. Fry the paste for about 5 mins until thickened, then add the chicken. Fry for 8-10 mins until the chicken starts to colour. Pour over the stock, bring to the boil and leave to simmer for 10 mins, stirring occasionally.
Check the chicken is cooked by cutting into one of the thighs and making sure it’s white throughout with no signs of pink. Season with black pepper, then add the sweet potato. Leave to simmer for a further 5 mins. Meanwhile, roughly chop the spinach and add to the stew. At this point you can leave the stew to cool and freeze for up to 3 months, if you like.
Scatter over the pumpkin seeds and preserved lemons, and serve with warm naan bread on the side.